This invention relates to station arresters and more particularly to station arresters including a stack of metal oxide varistors in which the capacitance of the varistors is graded so as to produce a more uniform distribution of voltage throughout the stack.
A station arrester is an electrical protective device which acts to protect transmission lines, transformers, and other equipment from excess voltage pulses arising from natural conditions such as lightning or from power system sources such as transient surges. Present station arresters typically comprise a stack of metal oxide varistors disposed in a cylindrical porcelain housing having external fin-like protrusions thereon for increasing the arc path length. The varistors are disposed in a stack; the top most varistor in the stack being connected to a line terminal of the arrester and the bottom varistor being connected to a ground terminal of the arrester. These terminals serve to electrically connect the arrester into the power system it is to protect. Metal oxide varistors are nonlinear resistive devices which under normal conditions exhibit a high resistance value; but which, when subjected to voltages in excess of their breakdown voltages, exhibit a very low resistance thereby acting as a protective device since destructive currents are shunted through the varistor stack. These varistors typically have high energy absorption capabilities and are well suited to act as protective devices. Nonetheless, in the station arrester application, the varistors are subject to the continual presence of the line power voltage. However, this line voltage is typically well below the varistor breakdown voltage. In this normal ambient state, the varistors exhibit a high resistance, but nonetheless some current does flow predominantly but not only by capacitive action.
Each varistor in the stack can be approximately modeled by a variable resistor in parallel with a capacitance, C.sub.v. Additionally, each varistor in the stack has associated with it a coupling capacitance to ground, C.sub.g. This group capacitance is approximately the same for each varistor in the stack. The capacitance C.sub.v associated with each individual varistor is an intrinsic property of metal oxide varistors produced from a sintered mixture of zinc oxide and various metal oxide additives. As a result of the capacitance effect, particularly the ground capacitance, a larger current flows through the top (i.e., line) varistors in the stack since the upper varistors also pass the capacitive ground currents which flow through the lower varistors. A larger current in these upper varistors subjects them to a greater level of power dissipation which results in a higher operating temperature and inferior stability for these varistor disks thereby acting to decrease the actual useful life of the arrester because of premature failures of the upper or line side varistor disks.
The conventional solutions to the nonuniform distribution of voltage and watts loss in the varistor stack require the tailoring of the capacitances of varistors by placing them in parallel with low dissipation capacitors. This method is expensive and cumbersome because it involves added costs associated with high voltage capacitors, connectors, and leads and, additionally, because of the requirement for additional space in the arrester housing, which space is expensive to provide because of the high cost of the housing itself. Nonetheless, the net capacitances are graded so that the varistor nearest the line terminal is connected in parallel with a capacitor having a greater capacitance than is the capacitance connected in parallel with a varistor closer to the ground terminal of the stack.